The hornet releases communication chemicals called attack pheromones. These pheromones will attract the other hornets and trigger their attack.
Be careful when you have stepped on a hornet as the other hornets will follow and attack you. In that case take your shoes off, leave them behind and run!
The nest of the European Hornet is generally built in an area that provides support for the nest instead of being free-hanging. Nests can be found in hollow cavities in tree trunks and in attics, barns and sheds. They may even be built in cavities in walls.
Another bad habit is that they kill bees.
If you have a problem with Hornets or any other pests click here for information on how to deal with them...(Link).